Photo by D. Burgess, from National Oceanic and Atmospheric Administration/Department of Commerce

Tornadoes scare me more than other disasters. As a child, I often spent summer vacations in Illinois. Although only 6 at the time, I still remember after-effects from the Palm Sunday tornadoes – my uncle’s house was neatly split in half and most houses on his street were simply missing. For years afterwards, I preferred to hang out in basements when the sky got that “tornado weather look”.

At the same time, a strange part of me loves storm chaser videos –the combination of crazy people and uncontrolled nature is irresistible. When a film lacks impressive destruction footage, I’m disappointed and have to remind myself that the houses belong to real people! So I decided it’s time to balance my warped fear/fascination with some reality. Besides, it’s tornado season!

Some of you might consider clicking away about now – “tornadoes don’t happen where I live”. I beg to differ. Many of us travel at some time to the Midwest or the South. Chicago and Atlanta are popular conference locations, even if not common vacation spots. Add in relatives and cross-country road trips – most people enter tornado territory at some time. Besides, everywhere is potentially tornado territory. It’s just a matter of odds. Except for Alaska and Rhode Island, every state had at least one tornado in the last 20 years.

How to make a tornado

Tornadoes are amazingly difficult to understand – they make earthquakes look easy! Just remember that warm air rises and cold air falls. When mixed together in the process, impressive winds result.

Supercell storm with wall cloud. Photo by Sean Waugh NOAA/NSSL

You start with a supercell –a special storm with warm moist air rising in a huge vortex (mesocyclone) from 1 to 50 miles wide. It’s often rain-free under the column because everything goes up, not down (“the calm before the storm”). This warm updraft climbs until it literally hits the atmosphere, where it chills and spreads into a giant anvil shaped cloud.

When the now-cold air spreads out past the vortex edge, it drops back toward earth as a downdraft. On the front side of the anvil cloud, the cold downdraft just brings lots of rain and hail. On the back side of the anvil, the descending cold air first shows up as a wall cloud. However, if this rear downdraft gets sucked back into the warm rotating updraft, not only do warm and cold air mix (bad), but they do so in a spinning pattern (really bad). With this combination and a sprinkling of bad luck – congratulations, you’ve made a tornado!

Watching the sky

So who cares? Now that I know what to look for, I hope I could recognize some of these cloud patterns. Although I’ll never equal a trained storm spotter, from now on, I’ll watch the sky when I visit tornado territory. Don’t worry – I’ll still depend on official warnings, but I like the concept of personal knowledge as well. I wonder if those who live in Tornado Alley pay much attention to the sky.

Speaking of the sky – here’s a well-known Tornado Myth: Green skies mean take cover – a tornado (or severe hail) is coming.False! Green sky and thunderstorms often go together but color doesn’t predict either tornadoes or hail. The green color results from combining red sunlight (often late in the day), heavy water content in the air (blue), and tall gray thunder clouds – but it doesn’t mean there are supercells or mesocyclones.

Meteorologists (weathermen) like tornado weather patterns. Their radar detects supercells and mesocyclones easily. The dangerous wall clouds create distinctive “hook echoes”. Modern radar can even show winds swirling into an actual tornado, although it can’t tell if the funnel has touched down yet. Sometimes radar picks up flying debris – that’s close enough to the ground for me!

The Storm Prediction Center in Norman, Oklahoma issues a “tornado watch” when weather conditions in a specific area indicate high risk of tornadoes. If they add “particularly dangerous situation”, this means a risk of multiple strong and/or exceptionally violent tornadoes. Take these words seriously – they’re used in less than 3% of watches. Once an actual tornado or funnel cloud is spotted by eye or detected by radar, the local weather service issues a “tornado warning”. If you hear this, TAKE COVER IMMEDIATELY – a real tornado is occurring Even without a tornado warning, watch for severe winds and hail – tornado weather equals severe storm weather! Just because a funnel hasn’t shown up, doesn’t mean that everything is hunky-dory.

If one tornado is bad, multiple tornadoes are worse. Multiple vortex tornadoes are several funnels simultaneously rotating around or inside a single main vortex. They probably explain occasional strange instances of sharply delineated damage – like cutting my uncle’s house in half. Tornado families are multiple tornadoes from the same supercell – they occur in quick succession in the same area. Tornado outbreaks are multiple tornadoes from one larger weather system, with tornado free gaps of no more than 6 hours.

How can you measure tornadoes?

Attempting to measure most things about tornadoes, such as wind speed, size, and strength, is an exercise in futility. Not only is it technically impossible (how do you measure overall wind speed in a rapidly moving cloud that starts anywhere?), but tornadoes change rapidly throughout their brief existence – what you measure one minute can be different a minute later. Sometimes, it is difficult to tell whether there was just one tornado or a multiple vertex or tornado family. It’s best to just think of tornadoes as the epitome of chaos.

Despite these limits, it’s still “fun” to know ballpark figures. Wind gusts vary from 65 mph to over 200 miles per hour, with rare gusts measured by Doppler radar at above 300 mph. Scientists downgraded wind speeds about 10 years ago, when they discovered that damage occurs at lower wind speeds than originally suspected. The width of the damage path indicates tornado width, and can be as small as 10 yards or over 2 miles across. Every website lists a different average width – 100-200 yards might be a reasonable guess. Since a typical neighborhood street is about 13 yards wide, it’s easy to see how a tornado takes out every house.

Tornadoes can skip or jump, making path length difficult to determine (many historical “long path” tornadoes were probably multiple vortexes or tornado families. A single tornado can stay on the ground for less than a mile to over 100 miles. The longest path was probably 219 miles. Most tornadoes dissipate in under 10 minutes, but occasionally persist for an hour or more. Can you outrun (or out-drive) a tornado? Their travelling speed averages around 30 mph but they can zoom up to 70 mph or slow down to almost stationary.

Since meteorologists can’t measure tornadoes, they instead rate them by damage – after all, that’s what people really care about. After the tornado, experts assess damage to human-built structures and vegetation and assign a rating. The 6 point Enhanced Fujita Scale goes from F0 (light damage, like broken branches, toppled shallow trees, and chimney or sign damage) to F5 (incredible damage with frame houses lifted off foundations and carried through the air, severe damage to steel reinforced concrete, automobile sized missiles flying through the air, and debarking of trees).

The good news? Only about 6% of tornadoes are F3 or worse, and less than 0.1% are F5 (F3 = severe damage with roofs and walls torn off houses, trains overturned, strong trees uprooted, and cars lifted from the ground). Real numbers might be even better than this – reporting of smaller tornadoes is probably incomplete! So when you see a classic twister heading straight for you, you can rest assured that statistically there’s a 94 % chance it will only cause light to moderate damage (but don’t hang around outside watching!).

Tornado Frequency by State. Diagram from National Oceanic and Atmospheric Administration/Department of Commerce.

Where and when do tornadoes happen?

Tornadoes happen almost anywhere in the world, but the United States wins hands down for severity and frequency – we have about 1200 a year ( the number is climbing recently, but this may just be better reporting). Even when you look at frequency per land area, the US still gets 5 times more tornadoes than the second place winner – Australia, and 40 times more per area than other countries. The Great Plains provide a great mixing bowl for cold air from Canada and warm air from the Gulf of Mexico, stirred together by dry air from the Rockies.

Within the United States, nowhere is completely safe – in 2010, we had a F2 tornado in Aumsville, Oregon, about 15 miles from my house. Most occur in Tornado Alley (the central plains) with 50-100 tornadoes each year per state. Florida also has a high frequency at 66 per year, but most are low severity. Dixie Alley (the southeastern states) also gets more than their fair share of tornadoes.

Another Tornado Myth – tornadoes do not strike big cities (particularly downtown areas). False! Ask poor St. Louis – they’ve been hit multiple times. It’s just odds. The total square mileage of urban area is very small compared to the vast area in between. In addition, states with highest tornado frequencies tend to be sparse in terms of cities. It’s a good thing, because a strong tornado in a densely packed urban area could be catastrophic– visualize every window in every skyscraper blasted on both people inside and down below.

Is there really a tornado season or a tornado time of day? Tornadoes occur most often in the late afternoon and evening, with the greatest number in April through June. This just reflects thunderstorm patterns in general, particularly in the central plains. But these are just statistics. In reality, tornadoes happen any time of year and any time of day. When it happens to your Oregon town in December, statistics don’t mean much.

Dixie Alley tornadoes are known for ignoring classic patterns – even though frequency is lower, severity and death rates are higher. Their tornado season is year-round, including late fall, winter, and early spring – unfortunately, this leads to public complacency. Dixie Alley tornadoes can happen at night – a truly frightening concept! Even in the daytime, their tornadoes are often difficult to see. The terrain is hilly and forested, and the tornadoes are often clouded in rain, unlike the classic supercell appearance of the plains. Finally, population density in the south is much higher, there’s a high percentage of manufactured homes (tornado death traps), and very few storm shelters. Bad news all around.

Stay tuned for Part 2

So that’s part 1 of tornadoes – the basic who, what, where, and whens. I’ll post Part 2 later this week (it’s mostly written already, so no huge posting gaps this time!). For healthcare folks (and the morbidly curious), I’ll talk about types of injuries and causes of death – “need to knows” for medical response planning. And we’ll get into safety – what you should do before and during a storm to keep yourself and your loved ones as safe as possible.

Stay safe,

Sheila Sund, M.D.

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Part of the training one receives as a Weather Spotter for NWS includes a chart to gauge wind speed. By examining the damage, meteorologists are able to estimate the wind speed of both straightline and cyclonic wind storms. Those estimated wind speeds are how the EF scale is actually applied. Otherwise, a tornado that touches down in the middle of a Kansas wheatfield, runs for a few miles across open pastureland, and then retreats, would always be an EF-0 storm, simply because there was nothing to damage, despite its actual strength.

Thanks for the info and clarification. In re-reading, I didn’t make it clear that meteorologists don’t just look at damage to human structures. They focus heavily on vegetation damage (grass, trees, etc), to determine tornado severity. We’re lucky there is so much open pastureland in tornado prone areas – otherwise the human consequences of stronger tornadoes would be much worse.

Of all the things I learned when researching the topic, one of the most fascinating to me as a scientist was the difficulty in measuring tornadoes themselves, particularly when they are so big and obvious! Dependence on “after-the-fact” consequences, instead of assessing the storm itself, at first seemed like “old-fashioned” science. It makes sense once you consider what a tornado actually represents, but it had never really occurred to me before. I guess I’m used to medicine where scientists study things down to the micron level (although we do our share of after-the-fact “guesstimating” as well!).

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Sheila Sund, M.D.

I am a physician from Salem, Oregon with a disaster fixation. Having left my previous specialties of neurology and hospice behind for now, I am instead focused on emergency preparedness, particularly healthcare response. Somehow the powers that be frequently forget that physicians will be an essential part of any response and recovery. I stick my head in everywhere to make sure we are included, and in the meantime, learn a lot about emergency response in general.
By Sheila Sund
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